Quantum key distribution (QKD) is a technology used to securely generate and distribute symmetric cryptographic keys. Unlike classical cryptographic methods, QKD security is provided by the laws of quantum physics and doesn't depend on computational complexity of mathematical problems. This means that QKD generated keys are secure even in the presence of an attacker or an eavesdropper with access to unlimited computing power. In other words, QKD solves the problem of cryptographic key exchange between two remote nodes with information-theoretic security. The establishment of a quantum key distribution network involves connecting two or more QKD enabled nodes in a point-to-point configuration. QKD networks are complex and require a layered approach in order to systematically describe hardware and software elements of the network and traffic flow between them. To analyze the security of QKD networks, all layers need to be taken into consideration. This paper aims to provide an overview and classify known security threats and attacks on QKD networks according to the layers of QKD network architecture, in order to detect which layers of the network need additional research efforts.